Radio communication systems based on, for example, Long Term Evolution (LTE) standardized by the Third Generation Partnership Project (3GPP) usually use, in the allocation of radio resources to mobile stations, proportional fairness (PF) schedulers capable of achieving fairness of throughputs among the mobile stations while ensuring appropriate radio capacities (NPL 1). Fairness of throughputs among mobile stations means to reduce the difference in throughput among the mobile stations. A PF scheduler allocates radio resources on the basis of allocation indices M(PF) each calculated by the use of an instantaneous rate and an average rate, which are expected upon allocation of the radio resources. Equation (1) is a formula for calculating M(PF). In Equation (1), i denotes the identification number of a radio bearer; k, the identification number of a resource block (RB); r, an instantaneous rate; T, an average rate; and a, a weighting factor. A radio bearer is a logical radio communication network established when a mobile station is to receive communication service, such as mailing or browsing service. An RB is the smallest frequency block unit for allocation that can be allocated to a mobile station in LTE. An instantaneous rate is the number of transmission bits per RB determined according to channel quality cyclically reported by the mobile station. In LTE, an instantaneous rate is usually set at a transport block size (TBS) per RB that can be transmitted in a modulation and coding scheme (MCS) determined according to the channel quality obtained by converting a channel quality indicator (CQI) included in channel state information (CSI) cyclically reported by the mobile station. An average rate is the average number of transmission bits per RB allocated to the mobile station and is calculated according to Equation (2). In Equation (2), ω denotes a weighting factor (0≤ω≤1), and δ is a variable taking one when the RB is an RB allocated to a mobile station i while taking zero when the RB is an RB other than that. When no RB is allocated to the mobile station, δ takes zero, whereby T is updated to a smaller value. Therefore, with Equation (2), an effective average rate taking allocation frequency into account can be calculated.
      [          Math      .                          ⁢      1        ]                                            M                          i              ,              k                                      (              PF              )                                =                                                    r                                  i                  ,                  k                                                            T                i                α                                      ⁢                                                  [                          Math              .                                                          ⁢              2                        ]                                                Equation          ⁢                                          ⁢                      (            1            )                                                                    T            i                    =                                                    (                                  1                  -                  ω                                )                            ⁢                              T                i                                      +                          ω              ·                                                ∑                  k                                ⁢                                  (                                                            δ                                              i                        ,                        k                                                              ·                                          r                                              i                        ,                        k                                                                              )                                                                                          Equation          ⁢                                          ⁢                      (            2            )                              
By allocating radio resources to mobile stations each having a large allocation index calculated according to Equation (1), allocation of RBs to the mobile stations resulting in a high instantaneous channel quality relative to the average channel quality is prioritized, thereby making it possible to increase radio capacities. In addition, since the average rate decreases when no radio resource is allocated, the possibility of allocation increases in the next allocation opportunity. This prevents even a mobile station having a low channel quality from losing an allocation opportunity, and fairness of allocation opportunities among the mobile stations is achieved. Therefore, it is possible to increase the lowest throughput of the mobile stations in a radio communication system.
In the 3GPP, LTE-Advanced is being considered as a radio communication system subsequent to LTE. In LTE-Advanced, expansion of the maximum system bandwidth to 20 MHz or more has been considered to enable high-speed and large-capacity communications compared with LTE. However, it is not easy to ensure a continuous wide-band frequency domain in the current situation where various frequency bands are allocated for various uses. In view of this, a technique called carrier aggregation (CA) is being considered for LTE-Advanced, CA being for securing a bandwidth of 100 MHz at maximum by aggregating multiple system carriers each having a bandwidth of 20 MHz or narrower to enable wide-band communications (NPL 2). Each system carrier used in CA is called component carrier (CC). Mobile stations capable of CA can transmit and receive data by using multiple CCs simultaneously enabling high-speed communications.
The number of CCs possible for a mobile station to use simultaneously depends on the reception environment of the mobile station. For this reason, in some cases, there may be mobile stations with different number of CCs that can be used simultaneously. An example of such a case is a scenario, as in FIG. 11, in which a single radio station controls two CCs having different frequency bands and the CCs use the same transmission power. Since propagation loss is larger in higher frequencies, in FIG. 11, a signal from the CC of high frequencies does not reach a mobile station located in an area distant from the radio station, and therefore, the mobile station can use only the CC of low frequencies. In another scenario, as in FIG. 12, a remote radio head (RRH), which is configured by separating only a transmission/reception function from the functions of the radio station, is provided additionally to each local area having particularly intensive needs of traffic, in a communication area formed by a radio station, and one of two CCs controlled by the radio station is allocated to the RRHs. This can effectively increase radio capacities at low cost. In this case, a signal from any of the RRHs does not reach a mobile station distant from the RRH, and therefore, the mobile station can use only the CC of low frequencies as in the preceding scenario. When the above-described PF scheduler is employed in such a scenario, each allocation index needs to be calculated for each CC by using the average number of transmission bits among the allocated RB as the average rate. Accordingly, fairness of allocation opportunities is achieved in each CC only among the mobile stations to which a signal from the CC is received. On a system-wide level, a mobile station having a small number of CCs that can be used simultaneously has fewer allocation opportunities than those for a mobile station having a large number of CCs that can be used simultaneously. Therefore, fairness of throughputs among mobile stations is not achieved with this technique.
In view of the above, an improved PF scheduler has been proposed, the PF scheduler being configured to calculate each allocation index by using the total of the average rates calculated for each of all CCs (NPL 3). Equation (3) is a formula for calculating an allocation index by the improved PF scheduler. In Equation (3), c denotes the identification number of a CC.
      [          Math      .                          ⁢      3        ]                                            M                          i              ,              c              ,              k                                      (              PF              )                                =                                    r                              i                ,                c                ,                k                                                                    (                                                      ∑                    c                                    ⁢                                      T                                          i                      ,                      c                                                                      )                            α                                                            Equation          ⁢                                          ⁢                      (            3            )                              
According to Equation (3), a mobile station having a large number of CCs that can be used simultaneously has a high average rate in the entire system band compared with a mobile station having a small number of CCs that can be used simultaneously, and is accordingly assigned a small allocation index. As a result, a mobile station having a small number of CCs that can be used simultaneously has more allocation opportunities than those for a mobile station having a large number of CCs that can be used simultaneously, in a CC of low frequencies. Therefore, using this scheduler can increase throughputs compared with the case of a PF scheduler configured to calculate the average rate separately for each CC. To increase throughputs further, a method of calculating an allocation index M in consideration of the number of CCs that can be used simultaneously (referred to as general method below) has been proposed (PTL 1). Equation (4) is a method of calculating an allocation index in the general method. In Equation (4), N(CC) denotes the number of CCs that can be used simultaneously. According to Equation (4), a mobile station having a large number of CCs that can be used simultaneously is assigned the allocation index M of a smaller value than that calculated according to Equation (3), whereby the mobile station having a small number of CCs that can be used simultaneously can have more allocation opportunities than those for the case of Equation (3). This consequently increases fairness of throughputs among mobile stations compared with the case of using Equation (3).
      [          Math      .                          ⁢      4        ]                                            M                          i              ,              c              ,              k                                =                                    1                              N                i                                  (                  cc                  )                                                      ·                          M                              i                ,                c                ,                k                                            (                PF                )                                                                          Equation          ⁢                                          ⁢                      (            4            )                              